Underwood appeared on the cover of the October 2005 issue of Playboy holding a football while wearing only bodypaint designed like a Oregon State Beavers football jersey and matching painted bikini briefs. Underwood was in the Girls of Pac 10 pictorial in the same issue.

Underwood appeared in episode #2 of E! network series The Girls Next Door, as a playmate hopeful of having her test shoot taken at Playboy Studio West. Subsequently, she was given the title of Miss July 2006. She has since appeared in several more episodes and attended events at the Playboy Mansion. In a season 3 episode aired on May 6, 2007, it was revealed that Underwood, along with fellow Playboy models Holly Madison and Alison Waite, previously worked for Hooters restaurants. Underwood worked at the Beaverton, Oregon location.

On June 7, 2007, a group of Oregon State University students taking a women's studies course, created a poster celebrating Underwood becoming Playboy's 2007 Playmate of the Year. The poster was hung outside Bexel Hall for a few hours before being taken down by Oregon State University staff. The poster was modeled on the "Achievement" posters that line Campus Way at the university. The poster read, "Sara Jean: First OSU Beaver Playmate of the Year, Playboy June '07, OSU CENTERFOLD, People, Ideas, Innovation." (Jodi Ann Paterson was the first Beaver to earn the title in 2000.)

Underwood appeared as a pirate wench in Epic Movie, along with Playmates Qiana Chase, Irina Voronina and Jillian Grace.

Sara added further to her Playboy resume in January 2008 when she posed for a Playmate Xtra as part of the Playboy Cyber Club.